This invention relates to a synthetic resin composition and to a method for its manufacture. The resin of this invention has good properties under high temperatures and remarkable resistance to flame, when exposed to either direct impingement of flame or to irradiated heat sources, producing little or no smoke. More particularly, this invention relates to polyhydric phenol-aldehyde resin systems.
The plastics industry has enjoyed tremendous growth for decades as new uses for plastic were found. The construction and aircraft industries, the marine industry, the transportation industry, and others, increasingly use plastics to replace metals, wood, ceramics, plasters, and other conventional construction materials. Many families of plastic resins have been used, including polyvinylchlorides, polyethylenes, polypropylenes, polyester resins, melamines, and urethanes; and the resins have been combined with various filler materials such as wood flour, walnut shell flour, glass beads, ceramics, and carbon. The filled or unfilled resins have been foamed and have been combined with glass fibers, where strength characteristics were desired. Various combinations have been used to provide different characteristics,--strength, chemical resistance, corrosion resistance, aesthetic appearance, and low cost.
However, plastics have generally had a major deficiency. Plastic resins are organic compounds, some of them hydrocarbons, which are either readily inflammable or produce large volumes of offensive smoke when subjected to heat or fire. As plastics have found their way into the construction industries, there has been growing discontent and disenchantment on the parts of fire code authorities, fire departments, insurance companies, and those who concern themselves with public safety. In May, 1973 the Federal Trade Commission spoke of possible indictment of more than two dozen large plastic resin producers for misleading the public by allegedly falsely advertising their products as fire retardant.
In prior-art practice, organic resins have sometimes been halogenated in order to increase their resistance to heat and flame. Polyvinyl chloride, for example, contains much chlorine. Certain polyester resins have also been chlorinated in order to secure resistance to high temperature and to flaming. During exposure to flame or high heat, the molecular bonds have tended to break down, releasing chlorine gas, which does function as a flame suppressant, by blanketing the available fuel of the vinyl and depriving it of oxygen. However, during the decomposition process the hydrogen in the molecular complex combines with the chlorine, forming hydrogen chloride. In the presence of moisture, whether due to high humidity (which is produced during the combustion process) or due to sprinkler systems, or due to fire fighting efforts, the resulting wet vapors of hydrochloric acid are very corrosive, eating away at building structures and other surrounding materials and producing extensive damage, while also exposing those in the vicinity of the smoke to severe potential injury. With continued combustion and exposure to high heat, the hydrogen chloride can be converted into phosgene, an exceedingly poisonous gas.
Polyester resins sometimes utilize antimony oxides, antimony tri-oxides, boron compounds, or derivatives therefrom, as a snuffing agent, however, the greater the amount of additives used to improve flame resistance, the larger the quantity of smoke that is generated.
Various resins, such as melamines, polyesters, vinyls, epoxies, and urethanes, may also be filled with inert materials, such as clays, cements (e.g., alumina [aluminum hydroxide] and derivatives), asbestos, and mica, to reduce the quantity of resinous fuel available for combustion. Loadings of inert fillers often exceed 50%. High loading of inert filler materials, especially when used with halogenated resins and in combination with snuffing compounds, have been claimed to produce end products which are fire retardant. However, in actual field experience such claims have generally been shown to be illfounded and unsupported by results.
Field experience has shown that smoke generation is as important, or more important than, intense flaming because smoke generation of plastics and attendant problems have been very serious in fires in high-rise buildings and in aircraft, for instance. Such generated smoke has made it impossible for persons to find escape exits and has resulted in suffocation and death, while impeding the work of firefighters.
Almost four-dozen tests have been used to determine flammability characteristics of plastics, but many of these have proven inadequate or even misleading and only a small handful concern determination of the smoke-generating characteristics. One of these is the ASTM E84 test, known generally as the Steiner "Tunnel Test," and described in "Standard Method of Test for Surface Burning Characteristics of Building Materials . . . UL 723." A twenty-inch wide by 25 foot long cement asbestos board is placed on top of a U-shaped furnace 25 feet long and exposed to flame. The results are calibrated to equal zero. Then a piece of red oak wood, 20-inches wide by 25 feet long replaces the cement-asbestos board on the roof of a furnace, and the wood is ignited at one end by a gas burner. The time it takes for the wood to ignite and burn the length of the furnace is the "flame spread" and is used as a reference of number 100. The amount of smoke generated is measured by a photocell and plotted on a chart, the area under the developed curve being used to determine the total quantity of smoke generated. Conventional plastics when so tested under standardized conditions have flame spreads which range from 10 to 1500, and their smoke developed ratings ranging from 200 to 2000. Experience has indicated generally that the lower the flame spread, the higher is the smoke developed rating, progressing almost geometrically and inversely to the higher numbers.
A second test used to determine fire characteristics of materials is the Monsanto "small scale tunnel test." Instead of using a 25-foot-long test sample, the Monsanto test uses a 2-foot-long sample. It is generally conceded that the results obtained from the Monsanto tunnel test may be used to predict flame spread numbers that would be received in the "full scale" ASTM E84 Steiner tunnel test.
A third test to determine fire characteristics of materials is that developed by the National Bureau of Standards. It has been proposed for use by the ASTM, and is identified as the "E-5" test. The national Bureau of Standards test determines results both with direct flame impingement and indirect heat irradiation. During both tests (known as "flaming" and "non-flaming") the amount of smoke generated is monitored and plotted against time. It has been found that certain materials generate very large quantities of smoke with flame impingement and little or no smoke during the non-flaming test; conversely, other materials produce little or no smoke during the flaming test, but generated large quantities of smoke during the non-flaming test. Test results of over 100 materials are contained in the National Bureau of Standards Building Science Series Bulletin No. 18.
A fourth test useful in determining the flame and smoke characteristics of materials is in use at Ohio State University. This test was developed in recent years by Professor Edwin Smith. A test sample of less than one square foot is inserted into a test chamber and exposed to flame. It is found that materials which contain high amounts of anti-flaming compounds often generate large volumes of dense smoke.
While there appears to be no correlation between these four tests (except between the Steiner tunnel and Monsanto tests), any of these tests may be used to exhibit fire response characteristics of plastic materials: the rapidity with which flame will spread along the material's surface (the heat release rates), and the amount of smoke that will be generated by materials when exposed to a heat source.
A further problem met with plastics materials relates to their degradation when exposed to heat. Generally, thermoplastics such as polyethylene and polyvinyl chloride start to deform physically at temperatures that vary from 140.degree. to 220.degree. F. These materials may not therefore be utilized where high ambient temperatures are the normal operating condition. Thermosetting plastics, such as common polyesters, begin to degrade in the vicinity of 300.degree. to 400.degree. F. Phenolics, in combination with other materials, may remain physically stable to upwards of 500.degree. F. All these materials are relatively low in cost and are readily available on the market; but all of them have very bad smoke-generating characteristics. Certain families of materials such as polyamide-imides, when properly laminated and cured, exhibit favorable fire-resisting characteristics in terms of high temperature resistance (900.degree. F.) with little or no smoke generation; however, these raw resin materials are expensive, and polyimides are expensive to fabricate.
While aldehyde resin systems of phenol and resorcinol have been known to possess high temperature strength, they have not traditionally been used in combination with fiberglass. Phenol-aldehydes, resorcinol-aldehydes, and phenol-resorcinol-aldehydes link through a condensation reaction. During the condensation, large quantities of water are given off. Roughly, for every mol of phenol, a mol of water is given off. This tends to create voids in fiberglass laminates not made under high pressure and temperature conditions. A number of other problems are posed in the application of these resin systems with fiberglass. Phenol resin systems have not tended to bond well to glass. Glass fibers are usually treated with salts and chemicals such as coupling agents which make the glass more flexible and improve the bonding of the glass fibers to resins. The water of a phenol-aldehyde condensation reaction tends to react with these salts or coupling agents on the glass fibers, causing degradation and embrittlement of the glass and interferring with the coupling agents in common use. Yet it is often desirable to use fiberglass in order to achieve strength and durability. It is also desirable to be able to mold fiberglass parts without the necessity of curing the parts at high temperatures and pressures.